why doesn’t more lanes help mitigate traffic?

Question

I’ve always heard it said that building more lanes doesn’t help but I still don’t understand why. Obviously 8 wouldn’t help anymore than 7 but 3, 4, or maybe 5 for long eways helps traffic filter though especially with the varying speeds.

In: Other

Answer 1

Two main reasons:

1. Induced demand: if you add more lanes, more cars will drive on that particular roadway. There’s a sort of self-balancing equilibrium: there’s a threshold of a certain level of congestion or speed, at which a certain amount of people will or won’t be willing to drive on that roadway. If you add more lanes, suddenly the previously too-congested roadway became acceptable to people who previously wouldn’t have driven on it, so they will drive on it now. This happens until again the roadway becomes congested to the point where more people aren’t willing to drive on it.

2. At a certain point, a highway’s throughput is limited by other features, like the capacity of off ramps (which typically are signaled intersections onto fixed-lane local roads), interchanges, etc. That means you can’t keep increasing the lane count infinitely and expect throughput to go up infinitely. There are downstream bottlenecks. If you could also widen interchanges infinitely, and widen local roads onto which people get off the highway onto, then sure, maybe. But off ramps tend to only have 2 or so left turn lanes and and 2 or so right turn lanes.

OTOH, increasing the number of lanes, while it might not improve the experience of individual drivers from their individual perspectives, *does* increase the *overall throughput* of the roadway.

While the flux (measure of the amount of flow rate through a given cross sectional area of a surface) eventually stays relatively the same, the volumetric flow rate (the integral of flux) goes up because there is now more cross sectional area (more lanes) over which the same flux acts.

So overall, each car experiences the same speed, but there are now more cars, which means in a given day, more car-miles were moved through that highway.